1. Field of the Invention
The present invention relates to a new powder coating based on a combination of one or more organic polyhydroxy compounds with one or more organic polyisocyanates as crosslinking agents and to the use of this powder coating in the preparation of a coating on any heat-resistant substrate.
2. Discussion of the Background
Heat-curable powder compositions obtained by reacting a hydroxyl-containing resin with a masked polyisocyanate are known and enjoy widespread use. Among the masked polyisocyanates, .epsilon.-caprolactam-blocked isophorone diisocyanate adducts have become established as PU powder hardeners. The PU powders produced with these hardeners are taking up a prominent market position as a result of their superior weathering stability and thermal color stability.
For instance DE-A 21 05 777 disclosed .epsilon.-caprolactam-blocked adducts of isophorone diisocyanate (IPDI for short) and diols and/or triols as hardeners for PU powders, providing a range of advantages.
DE-A 25 42 191 teaches blocked polyisocyanates of IPDI, .epsilon.-caprolactam and a polyol which is a mixture of a diol and triol, and their use for the preparation (in combination with a hydroxyl-containing polymer) of heat-curable pulverulent coating compositions.
DE-A 27 35 497 (U.S. Pat. No. 4,246,308) describes pulverulent PU coating materials comprising hydroxyl-containing polyesters, polyacrylates or epoxy resins, wherein the polyisocyanate component used for .epsilon.-caprolactam blocking comprises the isocyanatoisocyanurate and, if desired, oligomers together with monomeric IPDI.
In DE-A 27 07 656 (U.S. Pat. No. 4,150,211) a binder combination is disclosed that is similar to that in DE-A 27 35 497, the hardener component used being a partially trimerized IPDI/hexamethylene diisocyanate/toluenediisocyanate mixture whose free NCO groups are blocked with .epsilon.-caprolactam.
DE-A 30 04 876 discloses blocked polyisocyanates consisting of .epsilon.-caprolactam-blocked adducts of IPDI and polyols, the adducts containing 3.3-8 NCO equivalents per OH equivalent. PU powder coatings are produced with these blocked IPDI adducts.
DE-A 31 43 060 describes PU powders in which the hardener component comprises reaction products obtained by chain extension of IPDI with an aliphatic diamine and subsequent .epsilon.-caprolactam blocking.
With suitable hydroxy compounds, these .epsilon.-caprolactam-blocked IPDI derivatives give rise on stoving, to coating materials which couple ease of handling with the features of good levelling, high hardness and elasticity and good chemical resistance. A disadvantage of these PU powder coatings, however, is the high stoving temperature, which is about 180-200.degree. C. A further disadvantage of these PU powder coatings is to be seen in the removal of the .epsilon.-caprolactam, which, during the processing of the powder coatings, makes it absolutely necessary to take special measures to clean the waste air and to recover the blocking agent.
There has been no shortage of effort expended to reduce the high stoving temperatures by choosing other blocking agents. For example, oximes (DE-A 22 00 342, EP-A 0 432 257, U.S. Pat. No. 3,857,818), triazoles (DE-A 28 12 252), cyclic amidines (DE-A 27 44 721) and secondary amines (DE-A 34 34 881) have all been proposed as blocking agents with relatively low deblocking temperatures for PU powder coating hardeners.
The fundamental disadvantage, however, namely the content of blocking agents in the powder coating, has not been overcome.
This fundamental disadvantage of the content of blocking agents in the powder coating has been eliminated by using linear IPDI derivatives which are free from blocking agent, contain uretdione groups and urethane groups and also have, at the end, urethane groups, urea groups or free NCO groups (EP-A 0 045 994, EP-A 0 045 996 or EP-A 0 045 998). Unfortunately, the preparation of the hardener is a complex process requiring complex technology.
The same applies to the PU powder coatings of DE-A 36 24 775, in which the hardener component used comprises polyisocyanates which are solid below 40.degree. C. and liquid above 150.degree. C. and contain no blocking agents for isocyanate groups.